Why are flood maps important?

Text: Katri Isotalo, updated by Mikko Huokuna, Mikko Sane and Antti Parjanne

Flood maps show how likely it is that an area remains under water when there is a flood. The modelling system is refined as the raw data becomes more detailed and information about the impact of climate change increases.

Picture of a flood map.

 

The EU Floods Directive speeded up flood mapping in Finland. During the first six-year period, closed at the end of 2015, Centres for Economic Development, Transport and the Environment (ELY Centres) prepared flood hazard and floor risk maps for 21 significant risk areas. In addition to these, flood maps have been prepared for roughly 120 areas by the end of 2018. On the basis of flood maps, ELY Centres prepare management plans for flood risks. These flood maps are available in the flood map service maintained by the Finnish Environment Institute (SYKE).

The flood maps indicate what areas are at risk during floods of different magnitudes. The maps not only help to prevent flood damage and to improve communication, they also offer support in land use planning. For example, residential buildings should be located at least so high that an overflow of water can reach the area only once in a hundred years on average.

The lowest recommended building levels were updated in 2014. The previous recommendations dated back to 1998.

An elevation model offers key information

The flood maps prepared by ELY Centres are based on the National Land Survey’s DEM2 elevation model and numerical flow models based on cross-sections of channels. Observations of water levels and flows have been used to calibrate the model. Flood models were calculated using this data and the probabilities of floods were determined on the basis of data material.

Flood maps have been prepared for areas where inland flooding is more likely to occur. In 2017, SYKE and the Finnish Meteorological Institute (FMI) prepared a flood map for Finland’s coastal areas. Finland’s environmental administration is in charge of flood scenarios regarding rivers and FMI is responsible for those regarding sea areas.

Flood hazards are indicated by different probabilities of floods. Return period 1/5a means that the water level rises to the flood height once in five years on average. Flood maps have been prepared for floods occurring once in 5, 10, 20, 50, 100, 250 and 1,000 years.

“We were able to prepare flood maps fairly easily for more return periods than what is required in the Floods Directive”, says Mikko Huokuna from SYKE.

Flood hazard maps present the coverage and depth of floods. In addition, flood risk maps present the number of people living in the flood area, roads remaining under water and special areas, such as treatment and educational institutions, waste processing sites and culturally and historically valuable sites. The number of people and buildings was evaluated using the building and apartment register, which has been supplemented using building data included in the National Topographic Database.

“When we compared different material, we discovered that the differences in the number of buildings located in flood areas between these two registers are smaller than we expected”, Huokuna says.

The flood map service also includes maps of dam damage surveys and previous floods. The flood map service has been built using Esri’s ArcGIS Server technology. As the service must operate round the clock without any interruptions during a flood, there is a back-up system in place.

The flood map service’s users mainly include government officials and some of the largest municipalities in coastal areas, but it has also been used by consultants and regular people.

The Flood Centre issues forecasts and warnings

Alongside the flood map service, SYKE and FMI established the Flood Centre at the beginning of 2014. It is a virtual centre that communicates acute floods to other authorities and maintains the hydrological information service. Data is openly available on its website to everyone. SYKE and FMI produce these services in cooperation with ELY Centres and the rescue authorities.

Flood forecasts and warnings are based on weather forecast models, water level and weather observations, sea models and the watershed simulation and forecasting system (WSFS). Forecasts prepared using WSFS are based on real-time hydrological data obtained from more than 200 water level and flow monitoring stations.

SYKE is in charge of flood forecasts and warnings regarding watercourse flooding. It is also responsible for ice jam forecasts and map services. FMI is in charge of weather forecasts, sea water level forecasts and warnings, as well as heavy rainfall warnings.

SYKE and FMI are also prepared to create flood maps during acute flooding. According to Huokuna’s experience, a flood map can be created in a matter of hours using different geographic information material.

Changes in the sea water level in Finland’s coastal areas are affected by the total water volume in the Baltic Sea, i.e. water flows via Danish straits, air pressure and wind conditions regarding short-term changes in weather, and the characteristic variation in the water level of the Baltic Sea.

“Where the aim is to identify future floods in sea areas, we also need to consider the post-glacial rebound in Finland’s coastal areas, the rising of ocean levels as a result of global warming and its impact on the level of the Baltic Sea”, says Ulpu Leijala, research scientist at FMI. “When we are talking about an intense sea water flood, it should be kept in mind that such a flood always results from the joint effects of multiple factors.”

Closer cooperation between different authorities

The Flood Centre was established not only to offer better communication, but also to help different authorities to work closer together. However, SYKE and FMI continue to convey information and release notifications in their own services, particularly during acute flooding.

Cooperation with municipalities should also be closer. FMI issues warnings of stormwater floods, among others, while municipalities are in charge of planning related risk management activities. “The impact of climate change, such as storm- water, is one significant development area in the future”, Mikko Huokuna says.

In spring 2018, SYKE released a preliminary stormwater flood map for municipalities to help them to prepare initial assessments of stormwater flood risks. The preliminary stormwater flood map has been prepared for nearly all urban areas in Finland using a surface runoff model. Its initial information is mainly based on the National Land Survey’s DEM2 elevation model. Transboundary cooperation has been carried out in the Tornionjoki area between the environmental administration, FMI, the Finnish-Swedish Transboundary River Commission, and the municipalities of Tornio and Haparanda.

Climate change calls for more attention

During this second six-year period for flood risk management, the aim is to address any needs to update flood risk areas. In addition to assessing the impact of climate change, development areas include at least identifying ice and hanging dam floods and increasing cooperation with municipalities.

“One challenge and the most significant uncertainty factor in forecasting the rising of ocean levels is the future behaviour of ice sheets. Knowledge of climate change is increasing and any uncertainties in calculations related to the rising of sea levels are decreasing, which means that we continuously need to update long-term water level scenarios and the lowest recommended construction levels”, Ulpu Leijala says. Flood maps are available here and the Flood Centre is available here. Flood map material is openly available as geographic information material here.

The publication of flood maps is reflected in house prices

According to a study published by FMI in autumn 2015, the publication of maps of flood risk areas has had an impact on the prices of properties located in these areas.

The FMI study analysed the development of the prices of residential properties in the Helsinki region, Pori and Rovaniemi before and after the publication of flood risk maps in 2006–2010. The results show that the prices of houses located in flood risk areas increased less than those of houses in each location on average. At the same time, the prices of similar houses located outside direct risk areas increased more than the average level in each location. Therefore, house prices represent loss risks better than expected.

A database consisting of roughly 340,000 property transactions was built for the study. As a result of this large database, it was possible to distinguish the impact of flood risk maps from other impact using an econometric model. The researchers Athanasios Votsis and Adriaan Perrels stated that markets can act as effective transmitters of climate change information, provided that this information is easily available.

 

 

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